Abstract

Formation conditions of suevite-like impactites from an ∼100 m thick drill core sequence through the Cretaceous-Tertiary Chicxulub crater were reconstructed from empirical data obtained by petrologic and image analytical methods. The temporal evolution of the cratering process from the initial stage of excavation to the collapse of the ejecta plume is evidenced by the petrographic characteristics and modal composition of the suevitic rocks, including the size distribution and shape parameters of melt particles. Emplacement of the lowermost suevitic deposits likely started in the first minute after the impact by the passing ejecta curtain that interacted with the expanding ejecta plume. These ejecta deposits were capped by a tongue of coherent impact melt that was transported outward from the crater center during the collapse of the central uplift ∼5 min after impact. On top of this brecciated impact melt rock, the collapsing ejecta plume deposited air-fall suevites. The basal air-fall unit, Middle Suevite, may have been deposited due to a density current–like clumping of hot debris. With progressive cooling, regions of the ejecta plume were entrained in its collapse that produced vapor condensates, accretionary rims, and different oxygen fugacities. After cooling progressed, atmospheric conditions began to reestablish over the crater and turbulence decreased, supposedly after the first 10 min of initial ejecta plume collapse. This led to a winnowing out of fine matrix material and distinct sorting. However, due to aquatic reworking, only material that was deposited until ∼1 h after cessation of turbulent atmospheric conditions was retained.